A magnitude 6.3 earthquake in North Korea was likely the result of the country's sixth nuclear test, media reports said.

North Korean state media claimed that the blast was a test of a hydrogen bomb.

The test was estimated to have a yield of 100 kilotons, meaning a blast that was four to five times more powerful than the explosion in Nagasaki, Japan, in 1945, a South Korean defense official told the country's Yonhap News Agency.

Pentagon officials told that the US government would have no official response until after the US fully assesses what happened.

South Korea's presidential office says the security chiefs for Seoul and Washington have spoken.

The office says US National Security Adviser H.R. McMaster spoke with his South Korean counterpart, Chung Eui-yong, for 20 minutes in an emergency phone call about an hour after the detonation.

South Korea's Yonhap News Agency initially pegged the earthquake at magnitude 5.6, but the 6.3 reading came from the US Geological Survey.